A nickel coating having a satin finish is desired for various applications on account of its decorative appearance and low glare. Typical applications for satin finish nickel coatings include automotive parts and trim pieces, such as radiator grills and door handles; housings and trim pieces for various photographic and electronic devices, such as cellular telephones, portable video recorders and cameras; furniture components; and the like.
Satin metal coatings have been produced using several different methods. In one method, the surface of a metal substrate is blasted with an abrasive medium such as aluminum oxide to provide a textured surface, which is then electroplated with a bright nickel-plating and chromium electroplating. Another method involves deposition of a satin finish nickel directly without mechanical treatment. This method uses a typical nickel-plating bath, which is admixed with large amounts of insoluble powdery materials in the bath, such as kaolin, talcum, barium sulfate, etc., having a particle size of from about 0.1 to about 0.3 micron. These methods are not generally preferred because they are relatively expensive.
A heretofore preferred process of forming nickel deposits having a uniform satin finish involves the use of an electroplating bath containing a water soluble nickel salt, a primary polishing agent, and an amount of polyethylene oxide polypropylene oxide (PEOPPO) copolymer that is sufficient to form a finely divided dispersion in the bath electrolyte at the operating temperature. In order to achieve an acceptable roughness depth for the deposited coating, it is important that the PEOPPO is dispersed throughout the bath electrolyte in the form of finely divided droplets or micelles. A conventional freshly prepared bath electrolyte will typically exhibit the desired finely divided dispersion of the PEOPPO. However, after a period of time, the finely divided droplets or micelles will coalesce or agglomerate into larger micelles. As the average micelle size increases, the roughness depth of the deposited coatings increases. The stability of the PEOPPO micelles can be improved by adding stabilizing wetting agents such as branch chained alkyl sulfates or sulfonates. This will delay coalescence of the PEOPPO. However, if nothing further is done to counter the growth of the micelles due to coalescence, the roughness depth will increase to an unacceptable level. If this is allowed to occur, the coatings deposited on a substrate article will have an unacceptable roughness depth. Such articles will be rejected and discarded for unacceptable appearance. Further, the bath electrolyte may have to be treated to remove the larger micelles. Treatment of the electrolyte is undesirable because production is suspended while the electrolyte is being treated.
A method to control the uniformity of the satin finish is described in U.S. Pat. No. 3,839,165. The disclosed electroplating method involves continuously removing a portion of the electrolyte from the bath, passing the electrolyte through a heat exchanger in which cooled electrolyte is passed counter-currently and then through a cooling apparatus to cool the electrolyte below the turbidity point of PEOPPO alkylene oxide contained in the electrolyte, passing the cooled electrolyte counter currently through the heat exchanger and re-heating the electrolyte to the bath temperature, and adding the electrolyte back to the bath. During this process, the PEOPPO becomes soluble in the cool electrolyte and the desired finely dispersed micelles are regenerated upon heating the electrolyte back to the operating temperature. Although this method can substantially increase the useful life of the bath electrolyte, the method requires a large amount of energy to cool a portion of the electrolyte below the turbidity point of the alkylene oxide adduct and re-heat the electrolyte back to the bath operating temperature. Accordingly, a process of maintaining the required micelle size that uses less energy, and therefore is less expensive, is desired.